1. Field of the Invention
The present invention relates generally to the field of holographic data storage and, more particularly, to a system and method for providing gain and thresholding to a holographic data parallel recording and replication system incorporating independent angular address assignment.
2. Background of the Invention
Within a data storage system, data transfers can be attributed to the primary tasks of data storage and recall arising from user requests, but also to data management tasks within a data storage system. Data transfers arising from data management tasks represent an increasingly significant proportion of the total data transfer volume. Data management tasks may require retrieval, transfer and replication or copy of data and, in some cases, data re-arrangement prior to copy.
For data storage systems, data replication is a multi-step process, which typically involves sequential recall of data, writing of this data to a temporary buffer memory, and subsequent writing of this data back to a copy storage medium. This process is speed limited by device data rates and available system resources-including the bandwidth necessary to support these multiple data transfers.
A replication process involving additional re-arrangement of individual copy data storage addresses to form a different copy storage structure prior to copying is hereby also termed as content migration.
One exemplary data management task is the reclamation of storage capacity occupied by data which has no longer any useful value. Such data, hereby also termed as useless data, includes but is not limited to, data which has expired, is superceded, or is erroneous. The continued storage of useless archived data, which, for the most part, is stored on magnetic tape storage media, results in a decrease in the useful data capacity of the storage media, leading to wasted storage capacity. Because of tape load and positioning times, and the problem of matching storage location sizes, overwriting of random tape sectors containing useless data with new data is not an efficient solution. In fact, it is more efficient to copy all useful data to an empty tape storage medium in a single copying step. Copying all useful data to an empty tape, however, is both time and resource consuming as the procedure is based on a multi-step process that involves recalling all useful tape data, writing the useful data to a disk buffer and, subsequently, writing the data back to tape. It is estimated that 20–50% of data movement to and from real tape in virtual tape environments are due to this reclamation process.
For archive storage applications, holographic data storage permits high density storage by allowing a plurality of data pages to be stored within the same volume of a holographic data storage medium by recording multiple overlapping spatial gratings or holograms. Various multiplexing methods can be implemented to store the plurality of data pages within the same volume of a holographic data storage medium. Some multiplexing methods are based on Bragg selectivity, such as angle or wavelength multiplexing, where either the angle of incidence or the recording wavelength of a reference beam is varied. Due to its page-based data organization, holographic data storage and read-out is inherently page parallel, thus enabling higher data transfer rates.
A volumetric holographic data storage medium is organized into a plurality of spatially separate data sites with each data site having a plurality of superimposed multiplexed data units. A data unit represents the fundamental element used to organize and record data in a physical volume element, which in the case of page-based holographic storage is a 2-dimensional array of data bits also designated as a data page. For other types of media, the data unit may be a single bit. Examples of data site arrangements on a data storage medium include, but are not restricted to, a grid-like organization for a cubic holographic memory; or a periodic spacing along multiple concentric tracks, in the case of a holographic disk.
Low-cost, photopolymer-based holographic data storage media having a high storage density and a write once-read many times capability have recently been commercialized; and rewritable holographic data storage media is currently being developed. It is expected, however, that at least initially, any re-writing process will involve an erasing step, for example, by illuminating the medium at a different wavelength. Also, because high density holographic data storage generally involves recording a plurality of pages or data units in the same volume, the possibility of direct data overwriting for holographic storage is precluded, as the erasing process affects all multiplexed holograms within a same volume. As a result, a system that incorporates holographic data storage for data backup and archive purposes will also require some procedure for reclamation of capacity, and more generally for the retrieval and transfer of data prior to its copy onto another holographic data storage medium.
An important feature of holography is that it allows simultaneous optical retrieval of several data pages recorded in a holographic medium and their direct simultaneous recording onto a second holographic medium. The direct replication eliminates the sequential electronic detection, analog to digital conversion and electronic signal processing steps necessary for performing data signal regeneration, and enables offline data replication, thus freeing up storage system resources. By copying data within this highly parallel optical domain, the data rate limitations of today's serial data handling storage devices can be overcome.
Previous proposals, for example, as described in U.S. Patent Publication No. 20030161246, published on Aug. 28, 2003, and entitled HOLOGRAPHIC DISK RECORDING SYSTEM, for parallel replication of holographically recorded data deal exclusively with the problem of full content replication. The proposals include illuminating the holographic data storage medium with a broad reference beam, whose angle of incidence can be changed in order to recover all similarly angularly addressed data pages from multiple data storage sites. The optically reconstructed data signals are then conveyed by a suitable imaging system onto a holographic replication medium, also illuminated by a broad reference beam with the appropriate angle of incidence. The replication medium records multiple volume holograms constituted by refractive index and/or absorption gratings proportional to the spatial modulation of light intensity resulting from the interference between the optical signal beam and reference beam within the medium.
Although the above-described procedure enables parallel readout of multiple data pages and direct replication, it does not permit partial data selection, for example, selection of only useful data for replication. Since the broad illumination beam illuminates a plurality of data sites indiscriminately, both useful data and useless data are simultaneously reconstructed and subsequently replicated. Therefore, the procedure is unsuitable for applications requiring copying or replication of selected data from a volume of stored data.
The above-referenced, commonly assigned, co-pending application entitled “SYSTEM AND METHOD FOR PARALLEL SELECTIVE REPLICATION AND DATA ADDRESS REMAPPING OF DATA CONTENT STORED IN A HOLOGRAPHIC DATA STORAGE MEDIUM”, Ser. No. 11/021,667, provides a system and method for selectively replicating in parallel holographically stored data that enables data stored in one or more holographic data storage media to be selectively replicated in another holographic data storage medium, while, at the same time, providing the additional capability of independent remapping of the data being copied to a different copy data address. The system incorporates a multi beam controller system, disclosed in the above-referenced, commonly assigned, co-pending application entitled “SYSTEM AND METHOD FOR PARALLEL SELECTION AND RETRIEVAL OF DATA STORED IN A HOLOGRAPHIC DATA STORAGE MEDIUM”, Ser. No. 11/021,930, and which forms a multi-beam illumination system capable of independently controlling the angles of incidence of each of a plurality of beams, hereby used both as a selective parallel retrieval system and as part of a parallel recording system, allowing independent angular copy address assignment. The above described replication system enables fast offline management of holographically stored data, by enabling parallel retrieval of partial content and its direct simultaneous replication to a different copy data address on a target medium.
However, all above described parallel holographic replication processes suffer from limitations, due to the lack of a signal regenerating process. Thus, similarly to processes for photocopying paper documents, the copies (and copies of the copies) present a lower signal-to-noise ratio (SNR) than the original. Today's data storage systems provide electronic signal processing (typically by digital processes after an analog to digital conversion of the data has occurred) to maintain the integrity of the data copies by preserving its SNR. One alternative would be to apply this same type of electronic detection to every holographically stored data bit before recording it optically. This method, however, suffers from electronic transfer rate limitations described above.
The above-mentioned co-pending, commonly assigned patent application entitled “SCHEME CAPABLE OF PROVIDING GAIN AND THRESHOLDING TO THE PARALLEL RECORDING AND REPLICATION OF HOLOGRAPHIC MEDIA”, Ser. No. 10/934,185, filed on Sep. 3, 2004, the disclosure of which is hereby incorporated by reference, discloses the use of a resonant cavity for providing gain and thresholding to the holographic parallel recording of data. In the application, parallel recording is achieved by means of a single reference recording beam mutually interfering, within a holographic data storage medium, with multiple spatially separated data beams to affect the parallel recording of multiple holograms with identical angular addresses, the cavity being formed along the reference beam arm. The invention, while providing a method for parallel data regeneration during parallel recording thereof, only allows for the data to be replicated to identical angular addresses, without providing a means for independently assigning different recording addresses to the data being simultaneously recorded. Furthermore the configuration employed for forming the resonant cavity, which therein is placed in the single beam reference arm, is only suitable for a single beam or a plurality of spatially separated and plane wave beams impinging at normal incidence on the surface of the reflective elements, and must be modified when employing a plurality of beams in both recording arms.
It would, accordingly, be desirable to provide a method and system enabling the formation of a resonant cavity around a holographic data storage medium, for providing gain and thresholding in a holographic recording system, wherein both the information bearing light beam arm and the reference light beam arm of the holographic recording system comprise an array of optical beams, and wherein the reference light beam arm is comprised of multiple beams, each with a dynamically adjustable angle of incidence.